1. Field of the Invention
The present invention relates to a water-based ink suitable as an ink jet recording ink or the like, for which high print quality or image quality can be obtained on regular paper, recycled paper or coated paper, and moreover storage stability is excellent.
2. Description of the Related Art
With conventional water-based inks, a method has been used in which a dye is dissolved in water or a pigment is dispersed in water. As means for dispersing a pigment in water, a method using a surfactant, and a method using a dispersion polymer having hydrophobic parts and hydrophilic parts are known. Moreover, as methods in which the surface of a colorant is coated with a polymer, for ink jet printer inks, a method using microcapsules containing a dye ink, a method using a microcapsulated pigment obtained by dissolving or dispersing a pigment in a solvent that is insoluble in water and then emulsifying this in water using a surfactant, a method using, in a recording liquid, microcapsules containing a solution or dispersion of a sublimable disperse dye in a water-soluble solvent and at least one type of polyester, a method using a phase inversion emulsification reaction or an acidic precipitation method, and so on are known, and moreover studies are being carried out into ink compositions comprising colored emulsion-polymerized particles and a water-based material.
Various water-based inks have been proposed as described above, but there are few cases in which studies have been carried out toward obtaining long-term storage stability. Moreover, conventionally, an ethylenediamine tetraacetic acid salt has been used in most cases as a chelating agent for water-based inks. As an example in which a nitrilotriacetic acid compound is used, although not as a chelating agent, there is an example in which a dye and a metal coordination compound of Ni, Cu or the like are used, this being to obtain good light-fastness, with no bleeding, and high image density and character quality with an ink jet ink (see, for example, Japanese Patent Application Laid-open No. 9-239964). Moreover, there is an example in which a structure able to coordinate with a metal is introduced into fine polymer particles (see, for example, Japanese Patent Application Laid-open No. 11-349875). Alternatively, there are also examples in which a chelating agent is used in an image recording layer.
As described above, various studies have been carried out into forming pigment-type water-based inks. Pigment-type water-based inks contain a large amount of hardness components such as calcium and magnesium originating from the pigment, and in the case of use as an ink jet ink, hardness components such as calcium and magnesium will also be present on the ink vessel and channels. These hardness components affect pigment dispersion, whereby coagulation becomes prone to occurring. Moreover, conventional dispersions are generally unstable, and if a substance having a hydrophilic part and a hydrophobic part such as a surfactant, a 1,2-alkylene glycol or a glycol ether as used in the present invention is present, then there will be a problem of attachment and detachment becoming prone to occurring, and hence the storage stability of the water-based ink becoming poor. With ordinary water-based inks, a substance having a hydrophilic part and a hydrophobic part such as a surfactant or a glycol ether is necessary to reduce blurring on paper. With an ink in which such substances are not used, the penetrability of the ink into paper is insufficient, and hence there have been problems that the types of paper on which uniform printing can be carried out are limited, and the print quality is prone to dropping.
Furthermore, there has been a problem that if an additive that is used as an essential component in the present invention (in particular, an acetylenic alcohol type surfactant, an acetylenic glycol type surfactant, a silicone type surfactant, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, or a 1,2-alkylene glycol, or a mixture thereof) is used in a conventional dispersion, then long-term storage stability cannot be obtained, and hence re-dissolution of the ink is poor, and thus the ink dries, and hence clogging is prone to occurring at an ink jet head nozzle, a pen tip or the like.
Moreover, with a pigment that has been dispersed using such a dispersant, there has been a problem that residue from the dispersant remains in the ink system, and the dispersant does not contribute to the dispersion sufficiently, but rather detaches from the pigment, and hence the viscosity becomes high. If the viscosity becomes high, then the amount that can be added of the colorant (pigment etc.) is limited, and hence adequate image quality cannot be obtained, particularly with regular paper such as PPC paper. Moreover, in the case of using a dispersion of a pigment or the like, hardness components such as calcium and magnesium are present as components originating from the pigment or at places that inks contact with in the case of ink jet recording or the like. The dispersion of the pigment or the like will coagulate through such hardness components, and hence it is necessary to eliminate the influence of the hardness components by using a chelating agent. In general an ethylenediamine tetraacetic acid salt is used, but if the amount added thereof is high, then again dispersion will be affected and it will no longer be possible to obtain stability. It has thus been necessary to use a compound that has an effect as a chelating agent even in a small amount. Moreover, with materials that inks such as ink jet recording inks come into contact with the materials, it is necessary to avoid compounds that evaporate, sublime or the like and are thus harmful. Ethylenediamine tetraacetic acid salts are generally considered to be safe, but this only refers to the safety as is or the safety when in the form of an aqueous solution or the like; the safety has not been verified for the case that oxidation occurs during the drying process or through contact with OH radicals in the air with ink jet recording or the like. Moreover, ethylenediamine tetraacetic acid compounds are considered to have poor biodegradability. In particular, ethylenediamine tetraacetic acid itself (i.e. not in the form of a salt) has high toxicity, and is a PRTR designated substance, and moreover ethylenediamine, which is produced through the decomposition of ethylenediamine tetraacetic acid, also has high toxicity, and hence it is preferable to not use ethylenediamine tetraacetic acid, even in small amounts.